The present invention relates to a vacuum cartridge for an electrical protection apparatus such as a disconnecting switch, a switch or a circuit breaker, said cartridge comprising an enclosure of substantially cylindrical shape closed off by two end-plates, two contacts extending axially inside the enclosure, at least one of these contacts, the movable contact, being connected to an operating mechanism and mounted sliding between a closed position of the contacts enabling flow of the current and a position in which the contacts are separated and withstand the voltage between them, and at least one conducting shield arranged around at least one of the contacts.
In the most commonly used shield design, the contacts are surrounded by a single shield having the function of protecting the insulating parts from metallic projections and of guiding the equipotential lines to prevent dangerous dielectric concentrations. This shield surrounds the two contacts and is situated in the middle of the potential of the two contacts. Thus, in theory, the potential is distributed homogeneously between the two contacts both inside and outside the cartridge.
The distance between the shield and the contacts is chosen such that the interaction between the shield and contacts is smaller than the interaction between the contacts. This enables the electric field between the contacts and shield to be minimized compared with that which is present between the contacts. Risks of flashover between the contact and the shield are thus prevented.
These flashovers between the contacts and shield are extremely dangerous, for when such a flashover occurs, the shield temporarily goes to the potential of the contact (doubling of the potential on the shield) and the distribution of the potential outside is unbalanced with a distribution of 100% of the potential on 50% of the length of the external insulation. This situation can degenerate into an external flashover generating a risk of explosion and fire. The document DE10029763 is also known describing a cartridge designed to withstand higher voltages. These cartridges comprise several ceramics, a shield being designed to be placed at the junction between two successive ceramics to dielectrically protect the triple points and prevent metallization. In this embodiment, the shield surrounds the contacts at an optimal distance corresponding to the distance between the contacts.
The drawback of this type of cartridge lies in the fact that it presents a large diameter. In addition, the higher the application voltage, the larger the distance between the contacts and the length of the ceramics has to be. To avoid flashovers between the contacts and shield, the diameter of the shield also has to be increased.
This increase of the diameter of the shield is however detrimental in terms of cost of the switchgear units and of electrical behaviour.
Indeed, the diameter of the ceramics is proportional to the diameter of the shield, which gives rise to extra cost. Moreover, if an external insulation is provided around the cartridge, the diameter of the outside enclosure also increases with the diameter of the cartridge, which also generates additional cost.
Finally, in the case of use the cartridge in a three-phase switch comprising a shielding, the interaction between the phases for a given distance between the phases is greater the larger the diameter of the cartridges, resulting in a penalizing electrical behaviour.